1. Field of the Invention
The present invention relates to improvements in a planetary gear transmission disposed between an engine or motor and a drive wheel or wheels of a motor vehicle such as an automobile and a railway carriage.
2. Discussion of the Prior Art
Planetary gear type transmissions which have a plurality of automatically selected gear or speed positions are widely used for motor vehicles. Such planetary gear transmissions are required to be simple in construction and small-sized, provide a wide range of speed reduction ratio, and permit easy control of the shifting operations.
A typical example of the planetary gear transmission is disclosed in laid-open Publication No. 50-90853 of unexamined Japanese Patent Application. This transmission includes a first, a second and a third single-pinion planetary gear unit which are disposed in series, so as to provide five forward drive speed positions. The first planetary gear unit has a first sun gear, a first planetary gear, a first carrier and a first ring gear, and the second planetary gear unit has a second sun gear, a second planetary gear, a second carrier and a second ring gear. Similarly, the third planetary gear unit has a third sun gear, a third planetary gear, a third carrier and a third ring gear. The planetary gear transmission provides a comparatively wide range of speed reduction ratio, and all the forward drive speed positions can be established without requiring substantially concurrent disengagement of a clutch or brake and engagement of another clutch or brake. Usually, the concurrent disengagement and engagement of two or more clutches and/or brakes are difficult to control, and a vehicle engine to which the transmission is connected tends to race or the planetary gears of the transmission tend to be locked, upon shifting of the transmission from one position to another. Therefore, the elimination of the concurrent disengagement and engagement of the coupling devices results in easy control of the shifting operations of the transmission.
However, the conventional planetary gear transmission as described above suffers from a problem which arises from the arrangement in which the input torque received by the input member is first imparted to the second sun gear of the second planetary gear unit when the transmission is shifted to the first-speed position having the highest speed reduction ratio. The input torque is maximum in the first-speed position and the sun gear of each planetary gear unit has the smallest diameter. However, the second sun gear to which the input torque is imparted in the first-speed position should have sufficient strength for sufficient durability. Accordingly, the second sun gear should have a relatively large diameter and/or the teeth should have a relatively large width. Therefore, the transmission tends to be comparatively large-sized and massive, contrary to the recent requirement for compact and small-sized construction of the transmission.